Blend Sketch Layer     Change sketch layer blending mode to Soft Light (or Multiply or Overlay) and reduce opacity to best effect.

Duplicate the layer under the sketch at this point and merge the top layer with the newly created layer under it. You have four layers again

Select the top layer and add Hide All Mask (black) and set foreground color to white.
If you have some watercolor brushes, use those to paint white in the areas where you want the top layer with the sketch to show.
It often looks nice to let some of the image with the lines overlap into the lighter areas at the edges.

Switch to a black foreground color to break up lines here and there.

Adjust the opacity of the top two layers just a little if that improves the image.